Image display systems using a spatial light modulator (SLM) have grown increasingly popular for use in projection systems. One common type of light modulator is that of a transmissive LCD (liquid crystal display) panel. In this type of device, polarization of light through a pixel is rotated to turn each one on or off. Typically, switching speeds of an LCD array of pixels is slow enough to require three separate light modulators, one for each of three colors, such as red, green, and blue. A projection system like this has a number of drawbacks. One is that the incoming illumination beam must be linearly well polarized. In most projectors, where the lamp is unpolarized, approximately half of the light flux is lost and total brightness of the projector is diminished. A few projector designs can use all the polarization of the light, but this requires costly optics and a complex setup.
Another problem with typical projectors is that each pixel is distinct in its cell of an array. This leads to a “screen door” effect in the resulting image with noticeable borders between the pixels. Although recent designs have improved upon this problem greatly, the effect can still be easily seen in the final image. One other problem is the general requirement for three SLM panels. Not only does this greatly add to the total cost of the projector, but it also complicates the optical setup and requires precision alignment and recurring adjustments to keep the three colors perfectly aligned with one another. Ultimately the materials involved may also limit brightness from the projector. Additionally, over time, the liquid crystal molecules can break down under the high intensity of the projection lamps used, causing brightness and contrast to degrade over the life of the projector.
Several types of reflective SLMs have been developed to overcome these difficulties. One such type of SLM employs Liquid Crystal on Silicon (LCoS), a technique that uses similar technology to transmissive LCD displays except that the transmissive liquid crystal medium is laid over a metallic substrate and operated in a reflective mode. This technology also suffers the same burdens of requiring the incident light to be polarized and potential breakdown of the molecular structure of the liquid crystal materials.
A solution that has been quite successful in the last few years is the Texas Instruments DMD™. The DMD™ is an array of digital bistable mirrors that can be turned “on” to reflect the light to be imaged through a projection lens, or turned “off”, to direct the light away from the lens. One limitation of this technology is that it can only modulate the light beam along one axis, usually in a digital bistable fashion. Accordingly, there must only be a single input beam of light coming from a single direction. Although various shades of gray are available through pulse width modulation (PWM) of the mirror, this is fundamentally a monochromatic process that would require additional equipment if color projection were to be achieved.
For high-end systems, a three-modulator system is used. FIG. 1 is a top view of a portion of a conventional three-reflective-panel projection system (specifically, a 3-DMD™ projector 100) including the prism assembly for color separation. An incoming light beam 102 from a very intense projector lamp can be split into three colors in a variety of ways; in the most common case, a prism assembly 104 is used to separate the red, green, and blue components to one of three separate micromirror arrays (106, 108, 110). Each colored beam is modulated appropriately and the three images are then combined as they exit the prism assembly into projection lens 112. A system such as this can exhibit extremely good color fidelity in its output. Because modulation speed of a digital micromirror is normally much faster than an equivalent LCD panel, a very high number of different colors can be produced at each pixel. In addition, this type of projector is very efficient in its use of the lamp intensity. Because much of the lamp's color spectrum can be used for each frame, the projector is brighter at a given lamp power than other similar technologies.
Such a system has a number of drawbacks, however. One is greater complexity. Similar to a 3-LCD projector, the 3-DMD™ projector must be precisely aligned and maintained to ensure that all of its imaged pixels overlap properly. Because the system modulates each signal very quickly, the electronics between each DMD™ panel must be synchronized with sophisticated electronics. Furthermore, the cost of an individual DMD™ chip is more than that of a corresponding LCD panel. All of this leads to an extremely expensive projector. Most of these have been used in cinema projection, as cost has so far been prohibitive for use in home or business.
Thus, the majority of projectors using a reflective SLM sold have been ones with only a single modulator. The DMD™ can be modulated quick enough so that a single mirror array can modulate several different beams of color. Color separation and production is a crucial yet is one of the most challenging areas for single panel reflective modulators. In order for a single panel to produce a full color image, the various color portions of the image share the mirror array either in time, space, or both. The most commonly used approach is time-sharing using a rotating color wheel. This common type of projector is illustrated in FIG. 2A. In this projector 114, an incoming light beam 102 from projector lamp 115 is focused through a rotating color wheel 116. The color wheel is composed of a plurality of different colored filters. Each segment may be repeated around the wheel, and there may be segments where no filtering of the light occurs. For simplicity, a simple color wheel with only three equal segments is shown, one each for red 118, green 120, and blue 122, a close up of which can be seen in FIG. 2B. After the color wheel, the beam is normally directed into an integrator rod 124, which is either a hollow tube with interior reflecting surfaces or a solid glass rod that uses total internal reflection to keep the beam within it. The intensity profile of most projector lamps is nonuniform; multiple reflections inside the integrator rod homogenize these variations. The shape of the exit port of the integrator is chosen to match with the shape of the SLM for maximal use of the light. The light is then focused onto the single panel modulator 126 which may be a DMD™ or some other type of SLM, and is then imaged by the projection lens 112.
Generally, the color wheel is spun at a high rate of speed to allow for high frame rates in the video image. As the red portion 118 of the color wheel intercepts the beam, the SLM modulates the appropriate intensity for the red component of each pixel. A similar procedure is repeated for the green portion 120 and the blue portion 122 of the color wheel. To achieve even a minimum 30 frames per second, this color wheel is spun to at least 1800 RPM. However, at this speed range, quality of the image may be degraded. Many people can see a distracting “rainbow” effect, particularly when colored images are portrayed in motion. This effect can be ameliorated by either increasing the number of colored segments around the wheel, increasing the rotation speed of the wheel, or both.
While these measures make the color transitions appear smoother, they also require a complicated feedback mechanism to synchronize the color wheel position with the light modulation timing electronics. At the transition between different colors of filter, there is an area called the “spoke” where the beam intersects both colors of the filter. This portion cannot be used easily without introducing visual artifacts in the image. Although some attempts have been made to use the “spoke” region to obtain brighter secondary colors, it is difficult to completely remove the artifacts, and this technique desaturates the colors. At increased rate of transition of the spokes, effectively less light from the lamp is used for the image resulting in dimmer displays.
One improvement that has been made by Texas Instruments to projectors using color wheels is by using Sequential Color Recapture (SCR). In a system using SCR, the color wheel of FIG. 2B is replaced by a spiral-pattern wheel of closely spaced dichroic filters shown in FIG. 2C. FIG. 2C shows the bands as being spaced farther apart than in an actual device for clarity of illustration. As this color wheel 128 rotates, several different colored bands impinge on the SLM surface at the same time and scroll smoothly across it in a linear fashion due to the shape of the spirals. At each filter section of the color wheel, a portion of the light not passed by that section is reflected back into the integrator rod and subsequently back toward the SLM through its correctly colored filter section. This can lead to an appreciably greater light throughput for a given lamp intensity versus a simple color wheel approach. In addition, as the color segments are spaced very closely together, the “rainbow” effect in the resulting image is lessened.
Though this spiral-patterned wheel works well in practice, it requires highly sophisticated techniques to synchronize the slightly curved colored bands on the surface with the control electronics of the SLM. As with a standard color wheel, some of the extra brightness gained using this technique must be sacrificed to prevent the mixing of light from adjacent bands.
There are a number of other designs that have been made to achieve proper color separation for a similar SLM that share the color in a spatial manner. One design includes the use of dichroic beamsplitters to separate a beam into three component colors. These separated beams are directed through a rotating prism that projects the three colors into scrolling bands on the SLM surface. Another method involves a technique wherein three separated color beams are reflected by a rotating polygonal mirror where the colored bands that similarly scroll over the SLM surface. In both of these techniques, most of the intensity of each of the three colors are concentrated within its own band. Thus, a projector using this illumination technique could potentially be very efficient in its use of the lamp power. One drawback of these techniques is that the color bands formed do not remain spatially uniform as they scan over the SLM surface, but change size. Precision synchronization of the SLM to the moving and changing color bands can be extremely difficult. Both of these techniques may also cause more artifacts in the resulting image than with a color wheel system, as the rotation rate of a prism or mirror cannot be as fast as the wheel.
Another approach uses color bands that do not move on the SLM surface but change colors. This approach may include the use of two rotating dichroic beam splitters arranged as color wheels that appear similar to FIG. 2B. The wheels are mounted to the same rotating shaft and are offset by 120°. In each case, three beams are created, one each of red, green, and blue. Over one revolution of the wheel, each of the single beams switches through all three colors in such as way that red, green, and blue are always present in one of the beams. The three beams may be directed into three parallel integrator rods that are output adjacent to each other. The ends of the three integrator rods are then precisely focused onto the SLM surface. Although greater power can be obtained from this arrangement, this is optically very complex. Diverging light from the end of each integrator must be very accurately directed onto the SLM into a precise region separate from the next region. If the beam overlaps onto the adjoining region, there will be a visibly apparent bright line at the interface. If too great a gap is left between the regions, the dark gap will also be noticeable in the projected image.
In an alternative, the three variable colored beams are separated, put through separate integrator rods, and directed onto three separate regions of the SLM. Each region of the MEMS device is constructed so that its axis of rotation is different from that of the adjacent region. The various beams are aimed at the surface such that the “on” state for its region will point the light hitting that region toward the projection lens. Precise alignment is not as important for this arrangement, since overlapping light onto the next region will be reflected away from the projection lens whether the micromirrors in that region are set “on” or “off,” although overlap among the various regions is minimized to increase to total light intensity of the projected image.
Various improvements to micromirror display technology have been proposed, an example of which is depicted in FIG. 3A and FIG. 3B. FIG. 3B shows the micromirror of FIG. 3A without the mirror. In this example, the micromirror is supported not at its center, but along the corners of the mirror. This innovation decreases the amount of light scattering as compared to the DMD™ and strives to achieve higher contrast ratios. This method of support also allows the mirror size to be more readily decreased. Not only does this improve the resolution and pixel count of the mirror array, but it also allows the mirrors to tilt at steeper angle, further improving the contrast of such a projector. Nevertheless, this is still a bistable mirror. Thus, for a single panel system, some mechanical color separation technique such as a color wheel or others may remain necessary.
One common factor in all of these approaches is the requirement for rapidly moving mechanical parts to achieve color separation. These moving parts add complexity to the projector, requiring precise synchronization of the mechanical color separation system with the SLM electronics, while also exhibiting noise and reliability issues. Although heat buildup in projectors using high-intensity lamps requires the use of noisy cooling fans, motors to run the color separation element or elements add an appreciable amount of noise to the system themselves. Especially in the consumer market, a quieter system will always be preferable. In addition, these mechanical parts must always run during the use of the projector and be controlled very accurately. This reliance is problematic if the parts have a tendency to break down, or degrade in performance. Designing a projector to ensure reliability in this area increases costs. As such, there exists a need for a device that addresses the various deficiencies of the noted technologies.